The term Red Scare denotes two distinct periods of strong Anti-Communism in the United States: the First Red Scare, from 1919 to 1921, and the Second Red Scare, from 1947 to 1957. The First Red Scare was about worker (socialist) revolution and political radicalism. The Second Red Scare was focused on national and foreign communists influencing society, infiltrating the federal government, or both.

The first Red Scare began following the Bolshevik Russian Revolution of 1917 and the intensely patriotic years of World War I as anarchist and left-wing social agitation aggravated national, social, and political tensions. Political scientist, and former member of the Communist Party, Murray B. Levin wrote that the "Red Scare" was "a nation-wide anti-radical hysteria provoked by a mounting fear and anxiety that a Bolshevik revolution in America was imminent—a revolution that would change Church, home, marriage, civility, and the American way of Life." Newspapers exacerbated those political fears into xenophobia—because varieties of radical anarchism were perceived as answers to poverty. The advocates often were recent European immigrants (cf. hyphenated-Americans). Moreover, the Industrial Workers of the World (IWW) backed several labor strikes in 1916 and 1917 that the press portrayed as radical threats to American society inspired by left-wing, foreign agents provocateur. Thus, the press misrepresented legitimate labor strikes as "crimes against society", "conspiracies against the government", and "Plots to establish Communism".

Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths just past this range are called infrared (below red) and cannot be seen by the naked eye although they can be sensed as heat. In some cultures, certain shades of red are used to symbolize anger or aggression.

It is the color of blood when it is exposed to sunlight. In western countries, it is most commonly referred to as a color of evil, love, and sometimes, though rarely, happiness.

The word red comes from the Old English rēad. The word can be further traced to the Proto-Germanic rauthaz and the Proto-Indo European root reudh-. In Sanskrit, the word rudhira means red or blood. In the English language, the word red is associated with the color of blood, certain flowers (e.g., roses), and ripe fruits (e.g., apples, cherries). Fire is also strongly connected, as is the sun and the sky at sunset. Healthy, light-skinned people are sometimes said to have a "ruddy" complexion (as opposed to appearing pale). After the rise of socialism in the mid-19th century, red was used to symbolize and describe revolutionary movements.

Ferdinando Nicola Sacco (April 22, 1891 – August 23, 1927) and Bartolomeo Vanzetti (June 11, 1888 – August 23, 1927) were anarchists who were convicted of murdering two men during a 1920 armed robbery in South Braintree, Massachusetts, United States. After a controversial trial and a series of appeals, the two Italian immigrants were executed on August 23, 1927.

There is a highly politicized dispute over their guilt or innocence, as well as whether or not the trials were fair. The dispute focuses on contradictory evidence. As a result, historians have not reached a consensus.

Sacco and Vanzetti were convicted of the murders of Frederick Parmenter, a paymaster, and Alessandro Berardelli, a security guard, at the Slater-Morrill Shoe Company factory, on Pearl Street in Braintree, Massachusetts, during the afternoon of April 15, 1920. Robbers had approached the two men as they were transporting the company payroll into two large steel boxes to the main factory. Berardelli, who was armed with a .38-caliber, five-shot, nickel-plated Harrington & Richardson revolver, was cut down as he reached for his gun on his hip; Parmenter, who was unarmed, was shot twice: once in the chest and a second time — fatally — in the back as he attempted to flee. The robbers seized the payroll boxes and escaped by climbing into a waiting getaway car, a stolen dark blue Buick, which raced off with the robbers firing wildly at company workers nearby.

Joseph Raymond "Joe" McCarthy (November 14, 1908 – May 2, 1957) was an American politician who served as a Republican U.S. Senator from the state of Wisconsin from 1947 until his death in 1957. Beginning in 1950, McCarthy became the most visible public face of a period in which Cold War tensions fueled fears of widespread Communist subversion. He was noted for making claims that there were large numbers of Communists and Soviet spies and sympathizers inside the United States federal government and elsewhere. Ultimately, McCarthy's tactics and his inability to substantiate his claims led him to be censured by the United States Senate.

The term McCarthyism, coined in 1950 in reference to McCarthy's practices, was soon applied to similar anti-communist activities. Today the term is used more generally in reference to demagogic, reckless, and unsubstantiated accusations, as well as public attacks on the character and/or patriotism of political opponents.

Born and raised on a Wisconsin farm, McCarthy earned a law degree at Marquette University in 1935 and was elected as a circuit judge in 1939, the youngest in state history. At age 33, McCarthy volunteered for the United States Marine Corps and served during World War II. He successfully ran for the United States Senate in 1946, defeating Robert M. La Follette, Jr. After three largely undistinguished years in the Senate, McCarthy rose suddenly to national fame in February 1950 when he asserted in a speech that he had a list of "members of the Communist Party and members of a spy ring" who were employed in the State Department. McCarthy was never able to prove his sensational charge.